docs: def VFD_efficiency(P, load=1):
      r'''Returns the efficiency of a Variable Frequency Drive according to [1]_.
      These values are generic, and not standardized as minimum values.
      Older VFDs often have much worse performance.
  
      Parameters
      ----------
      P : float
          Power, [W]
      load : float, optional
          Fraction of motor's rated electrical capacity being used
  
      Returns
      -------
      efficiency : float
          VFD efficiency, [-]
  
      Notes
      -----
      The use of a VFD does change the characteristics of a pump curve's
      efficiency, but this has yet to be quantified. The effect is small.
      This value should be multiplied by the product of the pump and motor
      efficiency to determine the overall efficiency.
  
      Efficiency table is in units of hp, so a conversion is performed internally.
      If load not specified, assumed 1 - where maximum efficiency occurs.
      Table extends down to 3 hp and up to 400 hp; values outside these limits
      are rounded to the nearest known value. Values between standardized sizes
      are interpolated linearly. Load values extend down to 0.016.
      
      The table used is for Pulse Width Modulation (PWM) VFDs.
  
      Examples
      --------
      >>> VFD_efficiency(10*hp)
      0.96
      >>> VFD_efficiency(100*hp, load=0.2)
      0.92
  
      References
      ----------
      .. [1] GoHz.com. Variable Frequency Drive Efficiency.
         http://www.variablefrequencydrive.org/vfd-efficiency
      '''
      P = P/hp # convert to hp
      if P < 3:
          P = 3
      elif P > 400:
          P = 400
      if load < 0.016:
          load = 0.016
      return round(bisplev(load, P, VFD_efficiency_tck), 4)